Milk lipids, largely composed of triacylglycerols, exist as a dispersion of globules surrounded by a membrane derived from the apical plasma membrane of milk secreting mammary epithelial cells. Within mammary epithelial cells, cytoplasmic lipid droplets appear to be the immediate precursors of milk lipid globules. Cytoplasmic lipid droplets appear to originate as very small, triacylglycerol- rich droplets which bleb or bud from endoplasmic reticulum. These droplets are surrounded by a protein and polar lipid coat material derived from endoplasmic reticulum. Droplets appear to grow by fusion with each other, and the ultimate size distribution of lipid globules in milk may be controlled by the extent of this fusion. Calcium, a cytosol protein fraction, and gangliosides of the lipid droplet surface appear to be involved in fusion of these droplets. Objectives of the proposed study are to establish the mechanism by which droplets are formed and released from endoplasmic reticulum, to determine the nature and mode of action of factors which promote lipid droplet fusion, and to establish how lipid droplets are attracted to and interact with apical plasma membrane. Cell-free systems in which lipid droplets can be generated from endoplasmic reticulum and in which lipid droplet fusions can be induced have been developed. A cell-free system in which lipid droplet-plasma membrane interaction occurs will be developed. Through application of biochemical methods, origin, fusion and interactions of lipid droplets in these cell-free systems will be studied. Results obtained will be correlated with observations made through ultrastructural and immunocytochemical studies of tissue specimens. These studies are directed toward one aspect of the overall goal of this research program, which is to understand what controls the rate of synthesis and secretion of the constituents of milk.